Big-T-

Well id be inclined to say you must be tired when doing them m8 or you would be getting your 5x5's done without failing...Some people react different to workouts but maybe your expecting too much by way of feeling..Personally when I do my legs I feel totally n utterly wipped out but chest,back etc etc isnt near as bad..But its not to say you havent trained them correctly..Do you see a difference in your size/strength?? how much rest are you having between sets...Do you alternate your workouts?
It could be a combination of things m8...